Tremor for Quake is the follow-up to Head Games' Aftershock for Quake.
Unlike its predecessor which comes with a campaign consisting of three episodes, Tremor is merely a compilation of 500+ individual SP/DM maps (nearly a quarter of which are exclusive to this package) and 100+ QuakeC patches collected from the net, backed up by a basic front-end to launch the game with the selected add-ons.
Interestingly, both the new and the freely-available maps are decked in replacement textures, possibly to avoid a copyright infringement with id Software.